Mixing opioids can add to the high, unless one overpowers the other completely, but make sure the overall amount of opioid you are taking is not going to be dangerous for you. There is an opioid conversion chart linked in the header at the top of this forum - take a look
A lot of people don't get much from tramadol - it's not the most recreational opioid out there.. to get the most out of it it's best to take it in divided doses over a period of time, as this allows your liver to convert the maximum amount to o-desmethyltramadol, which is what is responsible for tramadol's opioid effects. Make sure you don't take more than 400mg in 24 hours however, as there is a risk of seizures.
15mg oral morphine is quite a small dose and it may not add a lot to the tramadol. However, 25mg hydrocodone = about 100-150mg tramadol, or 25mg morphine (all oral). 200mg tramadol + 15mg morphine would therefore be stronger than 25mg hydrocodone according to the conversion chart, but tramadol conversions are difficult due to the varying responses. How much do you usually take? As a rule it's much safer to start low and add more if needed